The purpose of this investigation is to study the physiological adaptations which occur in skeletal muscle as a consequence of exposure to hypoxia and to cold. Specifically the effects of hypoxia and/or cold on skeletal muscle fiber composition and on muscle capillarity will be determined by histological and histochemical techniques with the aid of stereology. In addition other parameters involved in 02 transport will be studied. The effects of changes in these variables will be studied by modelling mathematically the 02 transport to tissues, using nonlinear, nonsteady partial differential equations. The effects of maturation on these variables will also be studied.